1. Field of the Invention
The present invention relates to a microlens and particularly, to a microlens array sheet.
2. Description of the Prior Art
In general, a microlens widely used in a fine optical system is being applied to an optical pickup head, an image sensor, various display systems and the like. One of the application examples is a microlens array sheet having microlenses of a few millimeters (mm)˜tens of millimeters, which are disposed at a regular interval or a certain interval two-dimensionally.
The microlens array sheet is used for an image sensor, a panel of an LCD (liquid crystal display), a projection display system and the like, and is being increasingly applied to various display systems. For example, the microlens array sheet may be applied to a rear projection screen to magnify and project an image projected from a light source onto a screen, may be applied to an image sensor such as a CCD (charge coupled device) or a CMOS (complementary metal oxide semiconductor) imaging device, corresponding to a pixel of the image sensor to thereby improve sensitivity and reduce interference between pixels, or may be applied to a panel of an LCD to improve pixel contrast performance.
Hereinafter, the microlens array sheet which is applied to the rear projection screen will be described.
First, the projection display system is a system that magnifies and projects an image projected from a light source onto a screen to provide the image to viewers. The performance of the rear projection screen used in the projection display system is determined by various characteristics such as a gain, a viewing angle, a contrast, a resolution and the like. However, of such characteristics, a large optical viewing angle is required when a plurality of users watch the projection display system at the same time. If the viewing angle is small, a viewer placed at a lateral side of the projection display system cannot observe an accurate image.
FIG. 1 is a sectional view of a microlens array sheet applied to a rear projection screen in accordance with a prior art.
As shown, the microlens array sheet in accordance with the prior art includes a transparent substrate 1; a microlens 2 arranged on the transparent substrate 1; and a gap filling layer 3 formed on the substrate 1 and the microlenses 2.
The gap formed between the microlenses 2 is a main factor that causes deterioration in transmittance of the microlens array sheet. Therefore, in order to prevent such deterioration, the gap filling layer 3 is formed at the gap between the microlenses 2.
A fill factor of the microlens array sheet is maximized through the gap filling layer 3. However, because the gap filling layer 3 is formed not only on the surface of the microlens 2 but also on the surface of the transparent substrate 1 exposed between the microlenses 2 in a vertical direction in the process of forming the gap filling layer 3, a radius of curvature of each microlens 2 (microlens including the gap filling layer) becomes greater than an initially set value. Also, a sag height of each microlens 2 is lowered by a height (H) of the gap filling layer 3 formed vertically on the surface of the exposed transparent substrate 1. Accordingly, because of the gap filling layer 3 formed vertically on the surface of the exposed transparent substrate 1, a viewing angle of the microlens array sheet is decreased.
FIG. 2 is a sectional view which illustrates an optical path with respect to the microlens array sheet of FIG. 1 in accordance with the prior art.
As shown, a collimated beam 6 perpendicularly made incident upon the microlens array sheet 5 after passing through an optical system such as Fresnel Lens or the like is refracted at a certain angle by each microlens 2 provided at the microlens array sheet 5 and exits.
Because the radius of curvature of the microlens 2 is increased by the gap filling gap 3 (The radius of curvature of the microlens 2 means a radius of curvature of a microlens including the gap filling layer, and the gap filling layer is also a part of the microlens.), an angle of exiting light refracted at a lower end portion of the microlens 2 becomes smaller than an initially set value. Accordingly, it is difficult for a viewer to observe the projection display system from its lateral side.
Although the description has been made on the microlens array sheet applied to the rear projection screen, the decrease in viewing angle is also a problem of a microlens array sheet applied to different kinds of image sensors or display systems.
As described so far, the microlens array sheet in accordance with the prior art improves the transmittance by filling the gap between the microlens with the gap filling layer. However, because the gap filling layer is formed even on the transparent substrate exposed between the microlenses, a radius of curvature of the microlens is disadvantageously increased.
Also, as the radius of curvature of the microlens is increased, an angle of light refracted and exiting to a lower end portion of the microlens becomes smaller than an initially set value. Accordingly, a viewing angle of the display system applied to the microlens array sheet in accordance with the prior art is decreased.
A microlens array sheet in accordance with another prior art is disclosed in U.S. Pat. No. 5,555,476 issued on Sep. 10, 2004.